The global positioning satellite (GPS) system is used by a user with a GPS receiver to determine their position. The GPS system was designed for and is operated by the U.S. military. The GPS system consists of a number of satellites in approximately 12 hour, inclined orbits of the earth, each of which transmit continuous positional information regarding their position relative to the earth. The orbit altitude of each satellite is such that the satellites repeat the same track and configuration over any point approximately every 24 hours. In actuality, the satellite reaches the same point four minutes earlier each day. There are six orbital planes, each with four satellites, inclined at about fifty-five degrees with respect to the polar plane. This provides a system whereby between five and eight satellites are visible at a given time from any point on the earth.
Two positioning services are provided by the GPS system: the precise positioning service (PPS) which is reserved for military use and the standard positioning service (SPS) which is available for general use. The following description is confined to the SPS although some features are common to both systems. The SPS is intentionally degraded by the U.S. Department of Defense (DOD) to limit accuracy for non-U.S. military and government users. The selective availability (SA) bias on each satellite signal is different. The resulting position solution is therefore a function of the combined SA bias from each satellite used to determine a position. By measuring the propagation time of these transmissions and hence the distance from three satellites to a user's position, an accurate calculation can be made of the user's position in three dimensions. To make a valid positional fix, the propagation times of the transmissions must be measured to an accuracy of better than 100 ns and then these times must be facilitated to the satellite signals which each have timing marks at approximately 1 .mu.s intervals. However, each satellite's signals are synchronized to an atomic clock, the accuracy of which is not maintained by the normal user of the system. As a result, the user's clock is said to be in error (in other words, different from the time kept by the satellite) by a clock bias C.sub.B . By measuring the apparent satellite signal propagation times from four satellites rather than three, the redundancy can be used to solve for the clock bias C.sub.B of the user's system and the three accurate propagation times required to determine position can be calculated. The signal propagation times correspond to ranges of the user from the satellites related by the speed of light c. Prior to correction for the user's clock bias C.sub.B, the apparent ranges of the satellites are all in error by a fixed amount and are called pseudoranges.
The data transmitted by each satellite includes three sets of information, the ephemeris, the almanac and the clock correction parameters. The ephemeris includes detailed information about the satellite's own course over the next two hours, the almanac consists of less detailed information about the complete satellite constellation for a longer period and the clock correction parameters allow the user to correct for the GPS satellite's own clock errors. The satellite transmissions consist of a direct sequence spread spectrum (DSSS) signal containing the ephemeris, almanac, and the clock correction information at a rate of 50 bits per second (bps). In the case of the SPS, a pseudo random noise (PRN) signal which has a chip rate of 1.023 Mhz and which is unique to each satellite is used to spread the spectrum of the information, which is then transmitted on a center frequency of 1575.42 MHz. The PRN signal is known as a course/acquisition (C/A) code since it provides the timing marks required for fast acquisition of GPS signals and course navigation. The C/A code repeats every 1023 bits or one millisecond.
The GPS satellite signals received at a user's receiver have a bandwidth of approximately 2 MHz and a signal to noise ratio (S/N) of approximately -20 dB. In addition, since the satellites are each moving at a speed in excess of 3 km/s, the GPS signals are received with a Doppler frequency offset from the GPS center frequency. As a result, a stationary GPS receiver has to be capable of receiving signals with frequencies within a 4 KHz range from the GPS center frequency, and a mobile receiver (as is usually the case) has to be able to receive signals over an even greater range. To recover the data and measure the propagation time of the satellite signals, the GPS receiver must cancel or allow for the Doppler frequency offset and generate the C/A code relevant to each satellite. Initially, at least, this task can be very time consuming since to despread the DSSS signals, the incoming and locally generated code must be synchronized. To find the code delay, the receiver must compare the locally generated code and the incoming code at a number of different positions until the point of synchronism or correlation is found. With a code length of 1023 bits this comparison can be a lengthy procedure. However, once the frequency offset and the PRN code delay for each satellite are known, tracking them is relatively easy.
If pinpoint accuracy is required, a differential GPS technique can be used. Differential GPS can be used to more accurately identify a user's position by making propagation time measurements for a mobile receiver and for a fixed receiver at a known location, using the difference to more accurately determine the position of a mobile receiver. A fixed reference station GPS receiver, which knows exactly the position of its antenna and the ranges from its antenna to each satellite, is used to provide corrections to remote GPS receivers. The reference station GPS receiver measures the ranges to each satellite using the received signals just as if it were going to calculate position. Range errors are then calculated by subtracting the measured ranges from the known ranges. These range errors are then transmitted by the reference station as differential corrections to remote users. The remote users using differential GPS will receive both the GPS signals from the satellites and the differential corrections from the reference station. The remote user can then use the differential correction to correct errors in the received signals and more accurately calculate its position.
A basic GPS receiver and a differential GPS receiver can both only provide positional data through the satellite transmissions and corrections received by the GPS receiver. Using these transmissions the GPS receiver can calculate a user's position in three dimensions. However, neither the basic GPS system or the differentially corrected GPS system is capable of giving the user any information relative to or about their position. GPS receivers of the prior art also do not include a transmitter for transmitting information.
A positional system designed and maintained by Irimble Navigation, Inc. provides information relative to a user's position. A user with a GPS receiver uses the receiver to first determine their position. Once their position is determined, the user's receiver system, using a cellular phone, calls a dedicated number and is circuit-switched to the server. The user can then obtain information about their position and the surrounding area, from the server. Due to the high airtime charges, this system is very expensive to use.
What is needed is a positional system which inexpensively provides information about and relative to a user's position efficiently.